The present invention concerns a process for preparing a biphenol from an alkylphenol, especially a process for preparing a 3,3',5,5'-tetraalkyl-4,4'-biphenol by the oxidative coupling of a 2,6-dialkylphenol in the presence of a cupric compound.
It is well-known in the art that self-condensation products are obtained when a phenol compound is oxidized. The method employed in preparing diphenoquinones, biphenols or polyphenoxy ethers by oxidizing phenols is generally referred to as the oxidative coupling of phenols.
The self-condensation products resulting from these oxidative coupling reactions can be categorized as either those resulting from the carbon-carbon coupling or carbon-oxygen coupling of said phenols. Biphenols and diphenoquinones are prepared by carbon-carbon coupling in accordance with the following general reaction: ##STR1##
Similarly, polyphenoxy ethers are prepared by carbon-oxygen coupling in accordance with reactions such as the following general reaction: ##STR2## wherein substituents are omitted and n is an integer.
Whether carbon-carbon coupling or carbon-oxygen coupling, occurs depends on the substituents, the oxidizing agents or the catalyst systems employed.
A variety of materials, including metals and various salts, oxides and complexes have previously been disclosed as useful in promoting the oxidative coupling of phenols.
Processes which have been used in the past for the preparation of 3,3',5,5'-tetraalkyl-4,4'-biphenol have generally been two stage processes, involving oxidation of a 2,6-dialkylphenol to a corresponding diphenoquinone, and then reduction by hydrogenation of the diphenoquinone to a corresponding biphenol. The first stage may be carried out in the presence of forced air and optionally with a suitable catalyst or by the use of strong oxidizing agents, such as chromic acid or a ferric salt. Such processes are disclosed for example, in U.S. Pat. Nos. 2,900,417 to A. H. Filbery et al. and 3,491,128 to B. B. Dewhurst.
One stage processes for the preparation of 3,3',5,5'-tetraalkylbiphenol are disclosed by Randell et al. in U.S. Pat. No. 3,812,193 and by Kaeding in U.S. Pat. No. 3,247,262. In Randell et al., 2,6-diisopropylphenol is oxidized to the corresponding biphenol with a ferric salt of a non-oxidizing acid in an aqueous medium. It is also disclosed in Randell et al. that both the ratio of ferric ion to phenol and the ratio of water to phenol are critical and the process is specific to 2,6-diisopropylphenol. In Kaeding, alkylphenols including 2,6-dialkylphenols in molten state are oxidized with a cupric salt of an organic carboxylic acid at about 140.degree. to 225.degree. C. The cupric carboxylate is reduced to the cuprous salt during the reaction and this may be restored to the cupric state by oxidation. It is also disclosed in Kaeding that optimum yields of biphenols are obtained from alkyl phenols which have relatively bulky radicals, for example, isopropyl, tert-butyl, tert-amyl, and where the cupric carboxylate is sufficiently soluble in the molten phenol reaction mixture.
Other one step processes for the preparation of 3,3',5,5'-tetraalkylbiphenol are described by T. F. Rutledge in U.S. Pat. No. 4,070,383, wherein an alkylphenol is oxidized by contacting an aqueous mixture thereof with oxygen in the presence of a catalyst system comprising; (a) a copper compound, (b) an anionic surfactant, and (c) an alkaline material.
A method for oxidizing phenols in liquid form with oxygen in the presence of a small amount of catalyst, such as precipitated cupric oxide, is disclosed in German Pat. No. 536,277.
The present invention possesses one or more advantages over the prior art which may be summarized as follows.
1. A polluting discharge of aqueous effluent may be avoided. PA1 2. Organic solvents are avoided during the oxidation stage. PA1 3. The spent oxidizing agent may be readily separated and recycled for further use. PA1 (a) a surface area about 5 to about 50 m.sup.2 /gram, and PA1 (b) sufficient basicity to allow 4.0 grams of the activated cupric oxide in 100 ml of deionized water, to indicate a pH of about 7 to about 10.